The invention generally relates to a perforating charge case.
Perforating charges are small explosive devices that are used to create holes in an oil well casing and tunnels into a petroleum-bearing formation for purposes of establishing the flow of formation fluids into the casing. Referring to a cross-sectional view of a perforating charge 2 in FIG. 1, the perforating charge 2 typically includes three basic components: 1. a cup-shaped metallic case 4 that circumscribes an axis 1; 2. a liner 6 that resides inside the case 4, is generally metallic and typically is conical about the axis 1; and 3. an explosive 5 that is located inside the case 4 between the bottom of the case 4 and the liner 6 so that the explosive 5 surrounds the liner 6 on the liner""s outer convex surface. The perforating charge 2 may include a connector 3 to secure a detonating cord to the perforating charge case 4.
When the explosive 5 detonates, the explosive 5 collapses the liner 6 inwardly onto the axis 1 and forces the collapsed liner 6 toward the open end of the case 4 toward the rock formation to form a perforating jet. The perforating jet forms a hole in the well casing and a tunnel in the formation. Simultaneous to the formation of the perforating jet, the detonating explosive 5 expands the charge case 4 radially away from the axis 1 until the case 4 fragments into several pieces. The breakup characteristics of the case 4, such as the size and number of fragments of the case 4, depends on several factors, primary among these being the mechanical properties of the case material itself. The charge case 4 typically may be fabricated from steel, but as explained below, the case 4 may also be formed from die-cast zinc. Charge case fragments are generally termed xe2x80x9ccase debris,xe2x80x9d or more generally, xe2x80x9ccharge debris.xe2x80x9d
Several perforating charges may be spatially arranged in a pattern (a spiral pattern, for example) in a device called a perforating gun. The perforating charges are generally ballistically connected via a detonating cord or some other means. In general, two types of perforating guns exist: 1. a hollow carrier perforating gun 10 (see FIG. 2) that typically includes a steel pipe 12 that houses radially oriented perforating charges 14 and isolates the perforating charges 14 from the wellbore environment before detonation; and 2. a capsule gun (not shown) that is essentially a metallic strip or similar device onto which the charges are attached and are exposed to the wellbore environment before detonation. Referring to FIG. 3, for the hollow carrier perforating gun 10, the perforating charges 14 create several exit holes 16 in the pipe 12 when detonated. The diameter of each exit hole 16 may be about one fourth to one half inches, depending on the characteristics of the perforating charges 14 and other factors.
In a vertical well, one historical advantage of the hollow carrier perforating gun 10 is that essentially all case debris is retained inside the gun 10. Small quantities of case fragments may escape the hollow carrier perforating gun 10 through the exit holes 16 immediately after charge detonation, but most debris settles to the bottom of the gun 10. After detonation, the hollow carrier perforating gun 10 may be retrieved from the vertical wellbore via a wireline or some other arrangement. Some of the case debris may be small enough to fit through the exit holes 16, however, no mechanism exists to expel large quantities of this debris through the exit holes 16. Any debris that does exit the hollow carrier perforating gun 10 through the exit holes 14 falls under gravity into the xe2x80x9cratholexe2x80x9d below the gun 10 to the bottom of the wellbore.
In highly deviated or horizontal wells, however, the situation may be quite different. In this manner, many modern completions employ xe2x80x9cextended reach,xe2x80x9d or very long horizontal sections, that are perforated. Therefore, during and after perforating, the hollow carrier perforating gun 10 is horizontal. After perforating, the hollow carrier perforating gun 10 typically is retrieved to the surface after being dragged along a significant length of horizontal wellbore. During this retrieval, random rotation of the hollow carrier perforating gun 10 may occur. All this contributes to the significantly increased likelihood that charge debris may escape from the gun exit holes 16. Not only may more debris enter the wellbore than in a vertical well, the debris may create more significant problems than would be encountered in a vertical well. More specifically, the debris may xe2x80x9cbridgexe2x80x9d in the well casing, causing the hollow carrier perforating gun 10 to get stuck during its retrieval; debris may fall into (and block production from) any perforations on the lower side of the well casing; and any debris that does flow toward the surface may collect at bends or xe2x80x9cheelsxe2x80x9d in the well casing, for example, where a horizontal section of the well casing meets a vertical section. Also, any debris that flows through the well may cause significant damage to both downhole and surface equipment (valves, for example).
For purposes of addressing the debris problem, die-cast zinc may be used as a replacement for steel as a material to form the charge case 4. In this manner, zinc is more effectively pulverized than steel, and any zinc debris may be dissolved with acid treatment. While successful overall, a number of difficulties may be associated with zinc charge cases: 1. for a given design, charge performance is often sacrificed; 2. for a given sufficient exposure time, the zinc debris is known to react with certain completion fluids (CaCl2, etc.), forming a hard cement that adversely affects the completion; 3. the zinc alloys typically used may lead to significant energy-liberating reactions that lead to observed xe2x80x9cgun shockxe2x80x9d and cause significant damage to completions and equipment; and 4. zinc liquid/vapor that is deposited on the gun carrier inner wall during carrier strain or deformation may lead to liquid-metal embrittlement and increase the likelihood of gun failure (splitting of the gun, for example).
Thus, there is a continuing need for a perforating charge case that addresses one or more of the problems that are stated above.
In an embodiment of the invention, a perforating charge case is made by a process that includes forming a material into a shape for the perforating charge case and annealing the material.
In another embodiment of the invention, a perforating charge case is made by a process that includes cold forming a material into a shape for the perforating charge case. The cold forming produces additional recrystallization nucleation sites in the material. After the cold forming, the material may be annealed to decrease sizes of grains of the material to improve a ductility of the material to increase fragment sizes of the perforating charge case when an explosive that is placed inside the perforating charge case detonates.
In some embodiments of the invention, the charge case may be formed at least partially from copper, and in some embodiments of the invention, the charge case may be formed at least partially from a superplastic material.
Advantages and other features of the invention will become apparent from the following description, drawing and claims.